Geoscience Reference
In-Depth Information
Winter
mb
400
450
500
550
mT
600
cP
(cAw)
650
700
cA
mPk
Source
mT
(source)
750
800
mPw
850
900
950
1000
Figure 9.3
The average vertical temperature structure for selected air masses affecting North America
at about 45-50°N, recorded over their source areas or over North America in winter.
Sources: After Godson (1950), Showalter (1939) and Willett.
The stability produced by the effect of surface
cooling prevents vertical mixing, so further
cooling occurs more slowly by radiation losses
only. The effect of this radiative cooling and the
tendency for air-mass subsidence in high pressure
regions combine to produce a strong temperature
inversion from the surface up to about 850mb
in typical cA or cP air. Because of their extreme
dryness, small cloud amounts and low tempera-
tures characterize these air masses. In summer,
continental heating over northern Canada and
Siberia causes the virtual disappearance of their
sources of cold air. The Arctic Basin source
remains (
Figure 9.4A
), but the cold air here is very
limited in depth at this time of year. In the
Southern Hemisphere, the Antarctic continent
and the ice shelves are a source of cA air in all
seasons (see
Figures 9.2B
and
9.4B
). There are no
sources of cP air, however, due to the dominance
of ocean areas in mid-latitudes. At all seasons, cA
or cP air is greatly modified by a passage over the
ocean. Secondary types of air mass are produced
by such means and these will be considered below.
2 Warm air masses
These have their origins in the subtropical high
pressure cells and, during the summer season, in
the bodies of warm surface air that characterize
the heart of large land areas.
The tropical (T) sources are: (1) maritime
(mT), originating in the oceanic subtropical
high pressure cells; (2) continental (cT), either
originating from the continental parts of these
subtropical cells (e.g., as does the North African
Harmattan
); or (3) associated with regions of
generally light variable winds, assisted by upper
tropospheric subsidence, over the major con-
tinents in summer (e.g., Central Asia). In the
Southern Hemisphere, the source area of mT air
covers about half of the hemisphere. There is no
significant temperature gradient between the
